IR: Restore the old behavior of getDISubprogram

getDISubprogram was mistakenly thought to contain a bug: we thought we
might need to try harder if we found a DebugLoc we didn't find.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221044 91177308-0d34-0410-b5e6-96231b3b80d8